linux-kernel - Re: [PATCH] block: fix q->max_segment_size checking in blk_recalc_rq

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Date:	Thu, 24 Jul 2008 11:07:02 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
cc:	David Miller <davem@...emloft.net>, fujita.tomonori@....ntt.co.jp,
	jens.axboe@...cle.com, linux-kernel@...r.kernel.org,
	linux-scsi@...r.kernel.org, linux-parisc@...r.kernel.org
Subject: Re: [PATCH] block: fix q->max_segment_size checking in blk_recalc_rq_segments
 about VMERGE

> > For a dumb I/O card, you advertise SG_ALL capabilities, the IOMMU
> > is going to merge things as it would have anyways, and you have
> > code in the driver to advance SG entries after each "dumb I/O".
> 
> Not that dumb ... they just have a limited number of SG slots.  We
> wouldn't want to run them as spoon fed PIO because that really would
> kill performance.
> 
> > There is zero value to the vmerge code, the real gains are being
> > realized already.
> 
> There is value to me in my testbed, which I can't achieve any other way
> (except by buying different SCSI cards).
> 
> As I said, you can compile it out on sparc just fine.  I wish to keep it
> running for parisc, so I'll maintain it.  If it ever bit rots out of
> parisc like it has done for the other architectures, then feel free to
> remove it.
> 
> James

So try to #define BIO_VMERGE_BOUNDARY 0 for Pa-Risc and tell us what 
performance degradation do you see (and what driver do you use and what is 
the I/O pattern).

If you show something specific, we can consider that --- but you haven't 
yet told us anything, except generic talk.

Mikulas
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